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Mild steels were coated by hot-dipping in molten aluminum and aluminum-silicon baths at 700 °C for 180 seconds. Phase identification of the Fe-Al or Fe-Al-Si intermetallic compounds in the hot-dipped aluminide steel was carried out by using a combination of scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD) and electron backscatter diffraction (EBSD). The results showed that the aluminide layer of the hot-dipped aluminum steel possessed a thicker intermetallic layer than that of the hot-dipped aluminum-silicon steel which was composed of minor monoclinic FeAl3 and major orthorhombic Fe2Al5. The intermetallic layer of the hot-dipped aluminum-silicon steel was composed of not only the same FeAl3 and Fe2Al5 phases in the hot-dipped aluminum steel but also Fe-Al-Si intermetallic compounds of hexagonal Al7Fe2Si and tetragonal (Al,Si)5Fe3.

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Low carbon steel was coated by hot-dipping into a molten Al-10%Si bath. The high-temperature oxidation was performed at 700oC for 1 h to 49 h in air, air +100% H2O, and air + 30% ethanol under atmospheric pressure. An elemental composition distribution, morphologies of the aluminide layer and the oxide scale were characterized by OM, XRD, and SEM/EDS. After hot-dipping treatment, the coating layers consisted of Al, Si, FeAl3, τ5-Fe2Al8Si, and Fe2Al5. The results of high temperature oxidation tests showed the oxidation rate were parabolic law in three different atmospheres. The polyhedral τ1-(Al,Si)5Fe3 formed at a short time oxidation completely transformed to FeAl2 and FeAl due to the composition gradient and the chemical diffusion. The effect of water vapor on the oxidation resistance of the Al-Si coating may be attributed to increase in Al and Fe ions transport, leading to loss of protective aluminide layer by formation of iron oxide nodules on the coating surface and at interface between aluminide layer and the steel substrate.

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